An electrophotographic technology has found wide spread application not only in the field of copying machines but also in the field of various printers because it can provide an image of immediacy and high quality. As for the photoreceptor which is the core of the electrophotographic technology, organic photoreceptors using, as their photoconductive materials, organic photoconductive materials having advantages of entailing no pollution, being easy to manufacture, and the like, as compared with inorganic photoconductive materials, have been used. Usually, organic photoreceptors have an electroconductive substrate and a photosensitive layer formed on the substrate, and as such organic photoreceptors, there are known a so-called dispersion type photoreceptor having a single photosensitive layer obtained by dissolving or dispersing a photoconductive material in a binder resin; and a so-called lamination type photoreceptor having a plurality of photosensitive layers, obtained by laminating a charge generation layer containing a charge generation material and a charge transport layer containing a charge transport material.
By the use of an organic photoreceptor, an image formed by using the photoreceptor may have various defects in some cases due to a change of the use environment or the change of electric characteristics, etc. by repeated use, and in order to stably form favorable images, a method has been known to provide an undercoat layer containing a binder resin and titanium oxide particles between the electroconductive substrate and the photosensitive layer (e.g. Patent Document 1).
Layers which an organic photoreceptor have are usually formed by applying and drying a coating fluid having a material dissolved or dispersed in a solvent in view of high productivity. However, in an undercoat layer containing titanium oxide particles and a binder resin, the titanium oxide particles and the binder resin are present in a state where they are incompatible with each other in the undercoat layer, and accordingly the undercoat layer is formed by applying a coating fluid for forming an undercoat layer having titanium oxide particles dispersed therein. Heretofore, such a coating fluid has been commonly produced by subjecting titanium oxide particles to wet dispersion in an organic solvent by a know mechanical grinding apparatus such a ball mill, a sand grinding mill, a planetary mill or a roll mill over a long period of time (e.g. Patent Document 1). It has been disclosed that in a case where titanium oxide particles in a coating fluid for forming an undercoat layer are dispersed by using a dispersing medium, an electrophotographic photoreceptor excellent in charge/exposure repeating characteristics can be provided even under low temperature and low humidity conditions by the material of the dispersing medium being titania or zirconia (e.g. Patent Document 2). However, conventional technology still has various insufficiencies of performance in view of the image, the stability of the coating fluid at the time of production, etc., along with increasing demands for formation of higher quality images.
Patent Document 1: JP-A-11-202519
Patent Document 2: JP-A-6-273962